Fluid pressure controlled positioning device



Oct. 25, 1949.

Al J. BENT FLUID PRESSURE CONTROLLED POSITIONING DEVICE Filed April so, 1947 2 Sheets-Sheet 1 INVENTR Arhur J, Bent 66 @j ATTORNEY Oct. 25, 1949. A, J, BENT 2,485,805

FLUID PRESSURE CONTROLLED POSITIONING DEVICE Filed April so, 1947 2 sheets-sheet 2 '2a 84 ai 8065 65 ea e5 es? 75 '21 v2 l A/ /I u l l/J 74 89` i 86,-

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TTORNE Y Patented Oct. 25, 1949 FLUID PRESSURE CONTROLLED POSITIONING DEVICE Arthur J. Bent,

Penn Township,

Allegheny County, Pa., assignor to The Westinghouse Air Brake Company, Wilmerding, Pa., a corporation of Pennsylvania Application April 30, 1947, Serial N0. 745,075

This invention relates to fluid pressure controlled mechanism and more particularly to apparatus particularly adapted for positioning airplane wing flaps.

One object of the invention is the provision of an improved apparatus of the above type.

The use of aps and the like for increasing the lifting power of airplanes or for braking same is common practice, and experience has shown the need of means for automatically permitting retraction of these aps to or toward their inoperative position when the air pressure exerted against them becomes excessive and threatens their destruction.

It is another object f this invention to provide improved means for positioning airplane wing flaps positively to a plurality oi extended or operative positions and which will respond to excessive air pressure from the air stream of the plane exerted against said aps to allow said flaps to automatically retract toward their inoperative position in degreeproportional to the intensity of said pressure.

This object is accomplished in the present invention by providing a mechanism comprising an arrangement of multiple uid pressure cylinders comprising pistons adapted to be positively moved to a plurality of relative extended positions and held in each of said positions by pressure of fluid acting on said pistons. The pistons are arranged to be operatively connected to wing and ap in such a manner that air pressure, incident to movement of the airplane, exerted against said flap exerts a thrust against said pistons which is opposed by the pressure of uid in the cylinders. With the fluid acting on the pistons compressible, the pressure thereof may be overcome by an excessive thrust transmitted to said pistons from the extended ap so that said ap will automatically retract toward its inoperative position in proportion to the excessive increase in air pressure acting on said ilap.

Other objects and advantages will become apparent from the following more detailed description of the invention.

In the accompanying drawing, Fig. 1 is a diagrammatic View, mainly in section, of a, fluid pressure controlled mechanism or apparatus embodying the invention; Fig. 2 is a part sectional view of a portion of an airplane wing embodying the apparatus; and Figs. 3, 4 and v5 are views of the apparatus, shown in Fig. 1, but in three other positions thereof.

According to the invention, the apparatus comprises a casing I having formed therein three coaxially aligned power cylinders 2, 3, and 4. Cylinders 2 and 4 are of equal diameter while cylinder 3 is of larger diameter than cylinder 2. Walls 5 and 6 are formed in the casing which act as partitions between cylinders 2 and 3 and cylinders 3 and 4, respectively. A removable end wall 'I closes the outer end of cylinder 2, and a similar end wall 8 closes the outer end of cylinder 4.

A power piston 9 is operatively disposed in cylinder 3 dividing said cylinder'into a pressure chamber I0 adjacent to wall 6, and an atmospheric chamber II adjacent to wall 5. A piston rod I2 is attached to piston 9 and projects centrally therefrom through chamber II and a bore I3 in wall 5 for engagement with a power piston I4 of smaller diameter than piston 9 and operatively disposed in cylinder 2. Piston I4 divides cylinder 2 into pressure chambers I5 and I6 which are adjacent to end wall 'I and wall 5, respectively. A piston rod I'I attached to piston I4 extends centrally therefrom through chamber I5 and vout through a central bore I8 in end wall I and an eye bolt I9 is adjustably secured to the outer end of said rod. The cylinder 2 is longer than the cylinder 3 so that travel of piston I4 will be greater than the travel of piston 9.

A power piston 2U, of equal diameter to piston I4, is operatively disposed in cylinder 4, dividing the interior thereof into pressure chambers 2l and 22. A piston rod 23 is attached to piston 20 and projects centrally therefrom into chamber 22 and through a central bore 24 in end wall 8 and an eye bolt 25 is adjustably secured to the outer projecting end of said rod.

A plurality of valve assemblies 3U, 3l, 32, 33 and 34 are provided for controlling operation of the power pistons 9, i4, and 20. Each of the valve assemblies above enumerated is provided with an inlet chamber 35, a control chamber 35, and an exhaust chamber 3l which are formed in casing I in coaxial alignment. Each of the inlet chambers 35 is open by way of a casing passage 38 and a branch passage 39 to a fluid pressure supply pipe 4U which is normally adapted to be supplied with iiuid under pressure from any suitable source. Each of the exhaust chambers 3'I opens to atmosphere by way of an exhaust passage 4I. A casing passage 42 connects the control chamber 36 of valve assembly 30 with the pressure chamber I5 of cylinder 2 at the outer end thereof. Similar casing passages 43, 44, 45 and 46 connect the control chambers 33 of valve assemblies 3|, 32, 33, and 34, respectively, with pressure chambers I6, I0, 2| and 22 of cylinders 2, 3 and 4. An annular valve seat rib 41 is formed in the cas-- ing encircling an opening 48 connecting chambers 35 and 39 in each valve assembly. A supply valve 49 is disposed in each chamber 35 for cooperation with seat rib 41 for controlling communication through the opening 48. A member 59, also disposed in each chamber 35, is subject to pressure of a compression spring I which forces said member 50 into point contact with valve 49 for urging said valve toward its seat rib 41. The valves 49 and members 59 are of such dimensions with respect to the peripheral dimensions of chambers that uid under pressure is free to Ilow thereby when said valves are unseated. A guide member 52 is formed in the casing in each chamber 35 for accommodating a rod 53 slidably disposed in a central bore through member 52 coaxially with said chamber. The members 52 divide the chambers 3G into two parts which however are connected by openings 55 through said members. Each rod 53 is arranged to contact at its one end the respective supply valve 49 and at its opposite end a disk-shaped release valve 59 in the respective chamber 3B opposite to an annular casing shoulder 51 formed at the upper end of said chamber, as viewed in the drawing. A central bore 58 connects the lower end of each exhaust chamber 31 with the upper end of each control chamber 36. A valve actuating member 59 is operatively disposed in each of the exhaust chambers 31 which member 59 comprises a hollow rod portion 60 which projects through the respective bore 58 into the upper end of respective chamber 36 for seating against the respective release valve 56. A central opening 6i extends through each portion 99 from the lower end thereof to ports 92 which open into the exhaust chamber 31. A cam follower G3 in the form of a pin is attached to the upper end of each rod portion 69 and projects outwardly therefrom throughl an opening 64 in the upper ends of the respective chamber 31, the several followers 63 being arranged in line and in parallel spaced apart relation. A flange 55 is formed in each member 59 at the junction of the follower 63 and the portion 60 for accommodating one end of a compression spring 55 disposed in the chamber 31 for urging said member 59 upward, a distance limited by contact between its ange 55 and the portion of the casing forming one end of chamber 31.

Each of the valve assemblies 39 to 34 inclusive has a fluid pressure supply position in which position the assemblies 39 and 34 are shown in the drawing, and a fluid pressure release position in which position the assemblies 3l, 32 and 33 are shown in the drawing. The parts of each assembly assume the fluid pressure supply position upon depression of follower 93 and attached sleeve 89 against spring 66 to seat said sleeve against the release valve 56, for closing the opening 6i and thereby communication between chambers 36 and 31, and to displace said valve 56 and rod 53 to unseat the supply Valve 49 against opposition of spring 5l for opening the inlet chamber 35 to control chamber 36 and thence to a respective passage 42, 43, 44, or 46. The release position of any of the valve assemblies is assumed when the portion 89 of member 59 is disposed away from the release valve 56 urged thereto by action of spring 65 for opening control chamber 3B and thereby connected passages 42, 43, 44, 45 or 49, as the case may be, to exhaust chamber 31 by way of passage 6! in said portion 99, and the spring 5I is thereby rendered effective to seat the supply valve 49, closing the supply chamber 35 from the control chamber 35.

A cam assembly 61, extending over the projecting ends of followers 63 comprises a plurality of substantially cylindrical cam members 59, B8', and 68 encircling a central shaft 69 to which they are attached coaxially and spaced apart at desired intervals longitudinally therealong. The ends of cylindrical cam members 58, 93', 83 are tapered or curved into the shaft 59 -for slidable engagement with the ends of followers 93 for displacing same upon longitudinal movement of the cam assembly, as will hereinafter be described. A control piston 19 is attached to one end of shaft 89 to effect such movement in one direction. The piston 19 is operatively disposed in a control cylinder 1I, dividing said cylinder into a pressure chamber 12, and an atmospheric chamber 13 through which the shaft 69 extends. A control pipe 14 is connected to the pressure chamber 12 for conveying fluid under pressure thereto, and a compression spring 15 is suitably disposed in chamber 13 for urging the piston 19 in the direction of chamber 12.

The opposite end of cam assembly 61 is slidably supported in a bore 16 opening through an end wall 11 of a spring stop cylinder 18 which is open to the atmosphere. On the side of wall 11 opposite the cylinder 1| the shaft 59 is provided with a ange 88 one side of which is arranged to contact a surface 19 on said wall 11 for limiting movement of said shaft, the several cam members and the piston 19 in the direction of the right hand by action to spring 15. The opposite side of flange 89 is arranged to engage an annular stop member 8i which is slidably disposed in the cylinder 18 and urged by a com-- pression spring 82 to seat against an annular casing shoulder 83 spaced a desired distance away from the wall surface 19. A centrally projecting stop rod 84 extends from the end of cam assembly 61 for engagement with a disk-shaped stop member 85 normally disposed a certain distance away from shoulder v83. The stop member 85 is slidably disposed within a central guide sleeve member 86, one end of which is removably secured to a removable end wall 81 of cylinder 18, while the opposite end of said sleeve member 86 forms an annular shoulder 88 against which said member 85 is urged to a normal position by a compression spring 89.

Engagement of one or another of the cams 68, 68', 98 with one or another of the followers 93 of the valve assemblies is adapted to actuate the respective assembly to its fluid pressure supply position, while, when disengaged, the valve assembly is adapted to assume its fluid pressure release position, as will hereinafter be described in detail.

Eye bolts 9, 25 may be pivotally connected to two elements, respectively, adapted to be positioned one relative to the other, such as to a structural member of an airplane wing, and to an airplane wing flap, for example, as shown in Fig. 2. The entire mechanism is supported by the elements to which the eye bolts I9, 25 are pivotally connected, the casing i therefore is not fixed and the relative distance between said eye bolts determines the position of one element relative to the other. If an airplane wing is one element and an airplane wing I'iap the other, the wing flap may be assumed to be pivotally connected, as shown in Fig. 2, at its one end to said wing whereby the distance between the eye bolts I9, 25 will determine the angular position of said wing flap relative to said wing. It matters not which of the eye bolts I9, is connected to the wing or which is connected to the ilap.

Operation In operation, let it be initially assumed that pipe and thereby chambers 35 are supplied with fluid under pressure and that chamber I2 of control cylinder 'II is void of fluid under pressure, as a result of which, due to action of spring 'I5 the piston 'I3 and attached cam assembly 61 will be in their extreme right hand position defined by contact between the flange 80 and casing surface 'I9 and which may be considered to be a first stop position of said assembly. The cam members 6B, 68', 68" will thereby be so positioned as to displace the followers 63 of valve assemblies 30 and 34 so that said assemblies will be in their supply positions, while the remaining valve assemblies 3I, 32, and 33 will be in their release positions. With the valve assembly 30 in supply position, fluid under pressure will flow to the pressure chamber I5 of power cylinder 2 by way of the respective casing passage 42, while with the valve assembly 3l in release position the chamber I6 of cylinder 2 is open to atmosphere by way of casing passage 43. The pressure chamber I0 of power cylinder 3 is similarly open to the atmosphere by way of casing passage 44 and the valve assembly 32 which is also in release position. Likewise, pressure chamber 2I of power cylinder 4 is similarly open to atmosphere by way of casing passage and the valve assembly 33 in release position, while the pressure chamber 22 of cylinder 4 is supplied with iluid under pressure through passage 49 by the valve assembly 34 which is in supply position. Since chamber I8 of cylinder 2 and chamber I0 of cylinder 3 are both open to the atmosphere as above described, the pressure of fluid in the chamber I5 of cylinder 2 acting on one face of piston I4 will urge said piston and the piston 9 to their extreme right-hand limit of travel, as viewed in the drawing. Similarly, pressure of fluid in chamber 22 of cylinder 4, in absence of fluid under pressure in chamber 2| of said cylinder, will urge piston 29 against end wall 8. The distance between eye bolts I9 and 25 is now at a minimum, and if said bolts be considered to be connected to a xed wing structure and to a wing ap member, as previously assumed, the apparatus will now be conditioned to maintain the wing iiap in a retracted position in which it is shown in Fig. 2.

Now assume that it is desirable to lower or eX- tend the flap on the airplane wing a certain fraction -of its total travel according to the degree of increase of lift and drag desired for reasons of air.- plane operation as in landing or taking o under different loads and diierent weather conditions. Fluid at a certain pressure is supplied by way of control pipe 14 to the pressure chamber 12 of' cylinder II from a suitable control device (not shown). This pressure of iiuid in chamber 'I2 acts on piston 'I0 to overcome the spring 'I5 and move the cam assembly 6'I in the direction of stop cylinder 78 to a second stop position dened by contact of the llange 881 with the annular Stop member 8I, in which it is shown in Fig. 3, which stops said piston and cam assembly due to the force of spring 82 acting on said stop member. The cam assembly will then remain in this position until the pressure of fluid in chamber 'I2 of control cylinder 'II is increased suoiently to overcome the action of spring 82 or reduced suciently to allow spring 'I5 to become effective to return it to the original position.

During movement of the cam assembly 61 to the above mentioned second stop position, the cam members 68, 68', 68" of said assembly are thereby each moved the equivalent distance to a position, in which they are shown in Fig. 3, lwhereby the follower 63 of valve assembly 30 remains in contact with cam 68 to maintain said assembly in its supply position. No cam member is yet in position to displace follower 83 of valve assembly 3|, so that said assembly is yet in its release position. The cam 68 has been moved into contact with and displaced follower 63 of valve assembly 32 so that said valve assembly is in its supply position. Follower 63 of valve assembly 33 remains out of contact with any cam member, and cam member 68 remains in contact with follower 63 of valve assembly 34 so that both assemblies 33 and 34 yet remain in their release and supply positions, respectively.

Since in movement of cam assembly 61 to this second stop position only the position of valve assembly 32 has been changed, from its release to its supply position, only the status of pressure chamber I0' of power cylinder 3 is affected. The chamber I0 is now supplied with fluid under pressure from supply passage 38 by the Valve assembly 32 in its supply position and through the casing passage 44. Pressure of fluid is still effective in chamber I5 on piston I4 to hold said piston in Contact with rod I2 of piston 9, but since said piston 9 is of greater area than piston I4, the fluid at the same pressure in chamber I9 acting on piston 9 creates a differential in forces which moves piston 9 from its position as shown in the drawing in Fig. 1 to its extreme left-hand position, in which it is shown in Fig, 3, carrying the piston I4 an equivalent distance in the direction of chamber I5. The rod I1 attached to piston I4 and eye bolt I9 secured thereto is thereby moved to a position further extended from the casing I, and the distance between said eye bolt I9 and the opposite eye bolt 25 secured to piston rod 23, which latter eye bolt yet remains positioned relative to the casing as shown, is increased, thereby positioning the airplane wing flap, pivotally connected to said eye bolt I9, to a proportionate extended position and maintaining it in said position in opposition to pressure of air exerted thereagainst as said flap moves through an air stream.

Should it be desired to further extend the wing flap, pressure of iiuid in pressure chamber 'I2 of control cylinder 'II is increased via control pipe 'I4 sufficiently to cause a force to act on piston 'I0 which Will move the cam assembly 61 further in the direction of stop cylinder 'I8 against opposition of both springs 15 and 82 to a third stop position in which it is shown in Fig. 4, dened by contact of rod 84 with the stop member 85, the force of spring 89 on said member preventing further movement thereof.

During movement of cam assembly 5l from its second to its third stop position, only the position of followers 63 of valve assemblies 33 and 34 are changed, the positions of followers 63 of valve assemblies 39, 3| and 32 which control operation of power cylinders '2 and 3 remain as described in the second stop position of said assembly 6l. In this third position, the cam 68" is now displacing follower 63 of valve assembly 33 and has moved out of contact with the follower 63 of valve assembly 34 so that the assemblies 33 and 34 are now in supply position and release position, respectively. With the valve assembly 34 in release position fluid under pressure in cham-i ber 22 of power cylinder 4 is vented to the atmosphere by way of casing passage 46, while with the valve assembly 33 in supply position iiuid under pressure is supplied to chamber 2| of cylinder 4 by way of the casing passage 45. A differential in pressures of iiuid between chambers 2| and 22 in cylinder 4 across piston 20 is thus created which moves said piston and thereby the rod 23 and eye boit 22 (not shown in Fig. 4) to an extreme position in which said piston is seated against the end wall 8. The distance between eye bolts I9 and 25 is now further increased so that the airplane wing flap connected to eye bolt I9 as previously described will now be further extended outwardly of the wing.

Now assume that it is desired to effect movement of the wing flap to a fully extended angular position relative to the surface of the wing, by increasing the pressure of fluid in the pressure chamber l2 of control cylinder 'Il suiiiciently to overcome action of springs 15, 82 and 39 acting on piston lll, ange 883, and rod 84, respectively, the piston il) and cam assembly 67 are moved further in the direction of stop cylinder i8 to a fourth stop position, in which they are shown in Fig. 5, defined by contact between the end face of flange 80 and the outer end of guide sleeve member 86 into which the central rod 84 of cam assembly (il now projects. In moving the cam assembly 6'! from third stop position to its fourth stop position, cam member 68 remains in Contact with follower 63 of valve assembly 32, the cam member 68 remains in contact with the follower 63 of valve assembly 33, and the follower 63 of valve assembly 34 remains out of contact with any cam member. The positions of valve assemblies 32, and 33 and 34 are therefore the same in the fourth stop position of cam assembly ll as in the third stop position, thereof, i. e. supply and release positions, respectively. Conditions in chambers Ill, 2I and 22 of power cylin-. ders 3 and 4, as influenced by the positions of valve assemblies 32, and 33 and 34, therefore remain unchanged and the positions of power pistons 9 and 29 and attached rods I2 and 23 determined by said conditions also are not changed. In the fourth stop position of cam assembly 6l, in which it is shown in Fig. 5, however, the valve assemblies 3G and 3l are in, respectively, release position and supply position, due to movement of cam member t3 away from follower 63 of assembly 3G and simultaneous movement of cam S8' into displacement contact with the similar follower 63 of Valve assembly 3 l Fluid under pressure is therefore now vented from chamber l of power cylinder 2 and supplied to chamber I6 thereof by way of casing passages ft2 and 43, and the valve assemblies 36 and 3 I. The power piston I4 is now moved away from the projecting end of piston rod I2 to an extreme position seated against end wall I by a force created by the now existent differential in pressures acting on its opposite faces, so that the piston rod Il and attached eye bolt I9 (not shown in Fig. 5) are moved outwardly to a maximum extended position. The distance between extreme positions of eye bolts IS and is now at a maximum and the wing ap will be now at its maximum extended angular position relative to the wing.

Now if it is desired to return the wing ap from its maximum extended position, in which it is shown in Figs. 1 and 2, to its fully retracted position or to an intermediate position therebetween, fluid under pressure is released from the pressure chamber 'I2 of control cylinder 'II so that springs 89, 8'2 and 'l5 become effective to return the cam assembly to any one of its previous stop positions, dependent upon the pressure to which said iiuid is reduced. Since the effect on the liuid pressure controlled apparatus of reducing the pressure of fluid in chamber 'I2 of control cylinder 'II is just the reverse of the effect of increasing said pressure, it will be appreciated that the piston rods Il and 23 will retract into cylinders 2 and 4 in successive steps in exact reverse of previously described extension thereof.

Now assume that in any of the previously described relative positions of pistons i4, 9, and 20, the pressure medium is a compressible fluid which is acting on said pistons to maintain them so positioned against opposing thrust acting on eye bolts I9 and 25, as would be the case when said bolts are connected to wing and extended flap moving in an air stream since the air exerts a pressure against said ap. The pressure of compressible :duid can be so adjusted that the force it exerts on pistons I4, S, and 20, in any one of their relative positions, is sufficient to maintain the flap extended into the air stream under normal conditions of operation but insuiicient to maintain said ap so extended should the pressure of air exerted against the iiap become dangerously excessive. Under these conditions the compressible fluid acting on the above named pistons would act as a spring and allow the ilap to move out of the air stream a distance sufficient to reduce the force of air acting thereon to a desired safe degree and thereby prevent destruction of the flap. As soon as the abnormal condition of excessive air pressure subsides, the spring action of the compressible uid will actuate the piston or the pistons to automatically return the ap to its former extended position.

Summary It will now be seen that I have provided a relatively simple mechanism particularly adapted for positioning an airplane wing iap and the mechanism being controlled by compressible iiuid is yieldable to permit automatic adjustment of the flap for limiting to a safe degree the force thereon from the air stream.

Having now described my invention, what I claim as new and desire to secure by Letters Patent, is:

1. A four position fluid pressure controlled apparatus comprising in combination with a source of fluid under pressure, a rst cylinder, a second cylinder of greater diameter than said first cylinder arranged coaxially therewith and separated therefrom by a first partition, a third cylinder of diameter equal to that of said first cylinder arranged in alignment with said second cylinder and separated therefrom by a second partition, first piston means slidably disposed in said first cylinder dividing it into a first fluid pressure chamber and a second fluid .pressure chamber adjacent to said partition, a rst adjusting rod secured to said first piston means for movement therewith and extending therefrom through said rst fluid pressure chamber, second piston means of greater area slidably disposed in said second cylinder dividing it into a third fluid pressure chamber and an atmospheric chamber adjacent to said first partition, rigid means secured to said second piston means extending through said first partition for cooperation with said first piston means, means for limiting travel of said second piston means to a distance less than that of said first piston means, third piston means of area equal to that of said first piston means slidably disposed in said third cylinder dividing it into a fourth fluid pressure chamber and a fifth fluid pressure chamber adjacent to said second partition, a second adjusting rod secured to said third piston means for movement therewith and extending therefrom through said third fluid pres sure chamber, and valve means operative to one position to open said first fluid pressure chamber and said fourth fluid pressure chamber to said source and to open the second, third and fifth chambers to atmosphere, operative to another position to close the third fluid pressure chamber to atmosphere and open it to said source, to another position to close the fourth uid pressure chamber from said source and open it to atmosphere and at the same time close the fth fluid pressure chamber to atmosphere and open it to said source, and to still another position to close said first iiuid pressure chamber t0 said source and open it to atmosphere and at the same time close said second iiuid pressure chamber to atmosphere and open it to said source.

2. The combination as set forth in claim 1 including cam means for operating said valve means, fluid pressure means for actuating said cam means, and resilient means for defining a plurality of stop positions for said cam means.

3. A fluid pressure apparatus for varying in chosen increments the distance between two elements to be adjusted comprising first and second pistons connected to said elements and a third piston interposed between said first and second pistons in coaxial relationship therewith, said third piston being of greater area than said first piston, means for limiting the stroke of said third piston to a degree less than the stroke of said first piston, means for connecting said first piston to said third piston for movement thereby, said first piston being subject on its outer face to pressure of fluid in a rst chamber and on its opposite face to pressure of fluid in a second chamber, said third piston being subject on the face adjacent said first piston to atmospheric pressure and on the opposite face to pressure of fluid in a third chamber, and said second piston being subject on the face adjacent said third piston to pressure of iiuid in a fourth chamber 10 and on the opposite face to pressure of fluid in a fifth chamber, a plurality of valve means, one for each of said chambers, and each comprising a depressible element operable upon depression for effecting operation of the valve means to supu ply fluid under pressure to the respective chamber and operable upon relief for effecting operation of the valve means to release fluid under pressure from the respective chamber, a longitudinally movable cam rod extending over the depressible elements of said valve means, cam means associated with said rod operable in a normal position of said rod to effect operation of said valve means to supply fluid under pressure to said first and fifth chambers and to open the other of said chambers to atmosphere, said cam means being operable in a second position of said rod to effect operation of said valve means to supply fluid under pressure to said iirst, third and fth chambers and to vent said second and fourth chambers, and in a third position of said rod to supply iiuid under pressure to said first, third, fourth and fifth chambers and to vent said second chamber, and in a fourth position of said rod to supply uid under pressure to said second, third and fourth chambers and to vent said first and fifth chambers, a spring acting on said rod urging it to said first position, a control piston connected to said rod operable upon supply of iiuid under pressure to a chamber to move said rod from said first position to said second position, to said third position and to said fourth position, precompressed spring means in said casing arranged to become successively effective in said second and third positions to define such positions and to oppose movement of said rod by pressure of fluid on said control piston, and stop means engageable by said rod in said fourth position to define same.

ARTHUR J. BENT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 715,291 Potter Dec. 9, 1902 2,379,306 Larson June 25, 1945 2,393,503 Bosomworth Jan. 22, 1946 FOREIGN PATENTS Number Country Date 3,783 Great Britain 1914 

